In FIG. 1 is shown the configuration of a conventional optical wavelength division multiplexing access system. This configuration is disclosed in “A WDM-based optical access network for wide-area gigabit access services”, J. Kani et al., IEEE Communication Magazine, vol. 41, issue 2, S43-S48, February 2003 (document 1). The optical wavelength division multiplexing access system comprises: a center node (OSU) 50, a wavelength multiplexing/demultiplexing device (herein after W-MULDEM) 60 and a plurality of optical network units (ONU) 70-1 to 70-n. A multiplexing section between the OSU 50 and the W-MULDEM unit 60 is established by extending a downstream optical fiber 1d, for transmitting downstream optical signals from the OSU to the individual ONUs, and an upstream optical fiber 1u, for transmitting upstream optical signals from the individual ONUs to the OSU. Access sections between the ONUs 70-1 to 70-n and the W-MULDEM unit 60 are established by extending downstream optical fibers 2d-1 to 2d-n, for transmitting downstream optical signals to the individual ONUs, and upstream optical fibers 2u-1 to 2u-n, for transmitting upstream optical signals from the individual ONUs.
In this configuration, one wavelength band λd is allocated for downstream optical signals from the OSU to the ONUs, and one wavelength band λu (≠λd) is allocated for upstream optical signals from the ONUs to the OSU. Wavelengths λd1 to λdn, in the wavelength band λd, and wavelengths λu1 to λun, in the wavelength band λu, are allocated for the individual ONUs. Further, an arrayed waveguide grating (AWG) is employed as W-MULDEM means for multiplexing or demultiplexing optical signals having individual wavelengths.
Optical transmitter/receiver 51-1 to 51-n of the OSU 50 transmit downstream optical signals having wavelengths λd1 to λdn, in the wavelength band λd, to the individual ONUs. A downstream AWG 52 multiplexes the downstream optical signals. An optical carrier supply module (OCSM) 53 for an upstream signal collectively oscillates optical carriers that are used for upstream signals and have wavelengths λu1 to λun, in the wavelength band λu, that are to be transmitted to the individual ONUs. A WDM coupler 54 multiplexes the downstream optical signals and the optical carriers for upstream signals and transmits the obtained signals to the W-MULDEM unit 60 through the downstream optical fiber 1d. 
In the W-MULDEM unit 60, a WDM coupler 61 demultiplexes the downstream optical signals having the wavelengths λd from an optical carrier for an upstream signal having the wavelength λu. A downstream AWG 62 demultiplexes the downstream optical signals having the wavelengths λd1 to λdn, and an upstream signal optical carrier AWG 63 demultiplexes the optical carriers for upstream signals having wavelengths λu1 to λun. WDM couplers 64-1 to 64-n perform wavelength division multiplexing for the respective downstream optical signals having wavelengths λd1 to λdn, which are to be transmitted to the individual ONUs, and the optical carriers for upstream signals having wavelengths λu1 to λun. The resultant downstream optical signals and the optical carriers for upstream signals are transmitted to the corresponding ONUs 70-1 to 70-n along the downstream optical fiber 2d. 
An optical transmission/reception unit 71 in the ONU 70-1 demultiplexes a downstream optical signal having the wavelength λd1 from an optical carrier for an upstream signal having the wavelength λu1, and receives the downstream optical signal having the wavelength λd1. The optical transmission/reception unit 71 also modulates an optical carrier for an upstream signal having the wavelength λu1, and transmits the modulated signal as a return upstream optical signal to the W-MULDEM unit 60 through the upstream optical fiber 2u. The other ONUs perform the same processing. An upstream AWG 65 in the W-MULDEM unit 60 multiplexes the upstream optical signals, having wavelengths λu1 to λun, that are transmitted by the individual ONUs, and transmits the obtained signals to the OSU 50 through the upstream optical fiber 1u. The upstream optical signal is demultiplexed by an upstream AWG 55 of the OSU 50, and the obtained signals are transmitted to the optical transmitter/receiver 51-1 to 51-n that correspond to the ONUs.
As is shown in FIG. 1, the wavelength band λd (wavelengths λd1 to λdn) for the downstream optical signal and the wavelength band λu (wavelengths λu1 to λun) for the optical carrier for the upstream signal are arranged so that they do not overlap along the wavelength axis (or along the optical frequency axis; optical frequency=light velocity/wavelength). An AWG, a property of which is that wavelengths at an FSR (Free Spectrum Range) interval are multiplexed or demultiplexed at the same time, is employed as the downstream AWG 62 and the optical carrier AWG 63 for upstream signals that are provided for the W-MULDEM unit 60. When the wavelength interval for the downstream optical signal (e.g., λd1) and the optical carrier (e.g., λu1) for the upstream signal are designated to be the FSR, the signal and the carrier can be routed to the same port by the AWG, so that only one AWG is required. In this case, the WDM couplers 61 and 64-1 to 64-n are eliminated.
When an obstacle, such as the severing of a fiber, has occurred at the multiplexing section between the OSU 50 and the W-MULDEM unit 60, communication with all ONUs is disabled. Therefore, it is preferable that, in order to obtain a dual multiplexing section, an optical fiber for current use and a redundant optical fiber be provided, and a function for switching to the redundant optical fiber be furnished for the OSU 50 and the W-MULDEM unit 60.
In FIG. 2 is shown the configuration of a common dual system. Opposite transmission devices 81 and 82 are connected via a current-use optical fiber 83 and a redundant optical fiber 84. An optical signal, transmitted by an optical transmission/reception unit 85 of the transmission device 81, is branched, to obtain two signals, by a photocoupler 86-1, and the branched signals are transmitted to the transmission device 82 along a current-use optical fiber 83 and a redundant optical fiber 84. Then, one of the optical fibers (the current-use optical fiber in this case) is selected at an optical switch 87-1 for the transmission device 82, and the signal is received by an optical transmission/reception unit 88. The same process is performed for a signal transmission in the opposite direction.
When the multiplexing section between the OSU 50 and the W-MULDEM unit 60 is to be doubled by using the switching method that employs the optical switch shown in FIG. 2, a dynamic function (optical switch) is required for the W-MULDEM unit 60, which originally does not need such a dynamic function. Accordingly, new control means is required to control the switching in the W-MULDEM unit 60, and the entire system becomes complicated.
It is one objective of the present invention to obtain a dual multiplexing section between an OSU and a W-MULDEM unit without adding a dynamic function, such as an optical switch, to the W-MULDEM unit.